Paramecium, the lowly fellers

Genus Paramecium

Web page by JM Del Vecchio

Image from source #5, Paramecium is the lower organism.


Image from source #6


Over the years, Paramecium has been studied, described and photographed extensively. This genus is discussed in all introductory biology and zoology texts. At least nine species are known, of which four are relatively common. Frequently studied species are P. caudatum (cigar shaped) and P. multimicronucleatum (multiple micronuclei).


Kingdom: Protista

Phylum: Protozooa

Subphylum: Ciliophora

Class: Ciliatea

Order: Holotrichida

Suborder: Trichostomina (or Hymenostomina)

Genus: Paramecium


Image from source #7

Range & Habitat

Image from source #8

The Genus Paramecium is commonly found throughout the world, in fresh and marine water containing bacteria and decaying organic matter.

External Appearance

Image from source #5

Paramecium is a small unicellular organism. It is elongated and ranges in size from 120 to 300 microns. The posterior half is slightly wider than the anterior half and is bluntly pointed, while the anterior end is rounded. On its underside there is a large and long groove running about half the length of its body. The outer surface of the organism is covered with many hundreds of minute hair-like projections called cilia.

Internal Structures

Although small, Paramecium has a complex internal structure with several easily indentifiable features.

(B) is the external oral groove, which leads to the internal (A) gullet or buccal cavity, at the end of which the cytostome or cell mouth is located. (D) is the macronucleus, which is relatively large and located near the center of the organism, and controls most of the metabolic functions of the cell. (C) is the micronucleus, which lies partly within a depression on the oral side of the macronucleus, and is involved primarily in reproductive and hereditary functions. (F) are the conspicuous contractile vacuoles, usually one at each end, each surrounded by several radiating canals which collect water water from the surrounding cytoplasm. The contractile vacuoles serve a critical function of osmoregulation, as water tends to accumulate inside the cytoplasm due to osmotic pressure. These structures are absent in marine Paramecium. (E) are food vacuoles, which are formed and 'loaded' beneath the cytostome. The age of a vacuole can be estimated by the state of dissolution of its contents. Not shown in the diagram are the trychocysts, tiny rod like structures embedded in the exterior membrane of the organism. When stimulated, the trychocysts discharge their contents and form long threads, which have several functions, two of which are anchoring and defense.

Image from source #7


Paramecium swims forward propelled by the coordinated motion of the cilia. As it advances it rotates because of the oblique beat of its surface cilia. In addition, the cilia arround the oral groove beat more strongly than the cilia at the opposite side of the body, which makes it swerve from side to side as it rotates. The strong beating of the cilia around the groove creates a current which constantly draws toward the organism a sample of the water ahead. And so it goes, moving, sampling and shifting through the water. Paramecium responds to a variety of physical and chemical stimuli, usually by an 'avoidance' maneuver, always turning to the left, the side away from the mouth, and advancing again in the new direction. If it encounters an obstacle, it backs up, swerves 30 degrees to the left, and continues forward. In the presence of a strong stimuli, it may turn up to 360 degrees, until an avenue of escape is located.


Paramecium feeds on bacteria and decaying organic matter. As it swims or browses along, the inflow current created by the strong beating of the cilia around the oral groove allows it to draw particles from a distance. Some particles are rejected by counter beating of the cilia outside the opening to the groove. Other particles are rushed in and against the cytostome membrane, which ballons in until it seals off, creating a food vacuole. A single paramecium may ingest as many as five thousand bacteria each day in this manner.


Paramecium may reproduce by transverse fission (called asexual) or by conjugation (called sexual). The reference to 'sexual' is not quite appropriate as Paramecium has no sexual organs or sexual dimorphism whatsoever.

Reproduction by transverse fission takes place in favorable conditions, with plenty of nutrients and space. This is the only way in which Paramecium can increase its numbers. A transverse constriction takes place at the mid section, dividing the organism into two daughter units which are different. The 'lower' unit must regenerate an anterior end, and the 'upper' unit must regenerate a posterior end. During transverse fission the micronucleus divides first by mitosis, and the macronucleus divides later by amitosis, a simple constriction. Under ideal conditions Paramecium can reproduce by transverse fission two or more times per day. The image at right shows a conspicuous transverse fission in the upper left portion.

Image from source #9

Image from source #9

Reproduction by conjugation takes place under poor nutrient or water conditions, or between two different mating types. During this process two organisms join together by their cilia and then fuse their oral surfaces. A complex series of changes results in an exchange of micronuclei, which brings about an exchange of genetic material. The complete process may last up to forty eight hours and requires the use of a substantial supply of stored energy. Following conjugation there is usually a rapid set of binary fissions.

Who eats Paramecium?

Paramecium has an extraordinary predator, Didinium.

Although slightly smaller in size, Didinium exhibits a voracious appetite for Paramecium. It is barrel shaped, with its cytostome at the anterior end, and has a girdle of cilia that allows it to whirl rapidly through the water. On top of the cytostome there is a sharp horn. Didinium attacks Paramecium horn first. It then maneuvers its prey into a position in line with its mouth, ingesting the organism by opening its mouth to swallow the prey whole. It can swallow paramecia twice its size, repeating this remarkable performance as many as a dozen times a day. A pack of didinia can rapidly decimate a large colony of paramecia. The sequence below shows the process with photos from source #10.

1. Attack and secure

2. Maneuver and line up

3. Gulp !


Various protista links from source #11.


  1. Curtis, Helena. 1968 The Marvelous Animals Garden City, NY: The Natural History Press.
  2. Jurand, A. and Selman, G.G. 1969 The Anatomy of Paramecium aurelia London: Macmillan and Co. (Note: This particular source contains numerous outstanding light and electron micrographs, showing the various structures of P. aurelia in great detail)
  3. Jahn, Theodore L., and Jahn, Frances F. 1949 How to Know the Protozoa Dubuque, IA: Wm. C. Brown Co. ISBN 0-697-04828-4.
  4. Lytle, Charles F. 1996 General Zoology Laboratory Guide Boston, MA: WCB/McGraw-Hill ISBN 0-697-13669-8.
  5. Micscape Image Gallery, Protozoa Portraits - Amoeba, Paramecium, Colpidium
  6. Protist Images: Paramecium caudatum
  7. Micscape Article: Pond Life - Paramecium
  8. Micscape - Protozoa, collection and study
  9. Reproduction and Paramecium
  10. Micscape - Protozoa. Didinium the master feeder
  11. Protista Online Lesson

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